Skid control system

ABSTRACT

A compound anti-skid logic system in which a plurality of different logic circuits are operative in parallel to process input signals from a common source in accordance with the different logic of each circuit with the different circuits complementing each other so that at least one logic circuit detects a wheel lock or an impending wheel lock under widely variable road conditions and brake application pressures. The compound system is incorporated in a master module with additional slave modules used with the master module to control axles in addition to those controlled by the master module.

United States Patent 1191 Urban 0 Oct. 23, 1973 SKID CONTROL SYSTEM [75]Inventor: John A. Urban, Livonia, Mich.

Assignee; Eaton Corporation, Cleveland, Ohio [22] Filed: Mar. 17, 1971211 Appl. No.: 125,142

[52] US. Cl. 303/21 BE, 303/7, 303/20 [51] Int. Cl. B60! 8/08, B60t8/10, B60t 8/12 [58] Field of Search 188/181; 303/7, 9, 303/13, 20, 21;317/5; 324/160-162; 340/52 [56] References Cited UNITED STATES PATENTS3,574,415 4/1971 Stamm 303/21 EB 3,482,887 12/1969 Sheppard 303/21 BE3,602,553 8/1971' Cumming et al. 303/21 EB 3,495,882 2/1970Ste1zer.....'l 303/21 F 3,494,671 2/1970 Slavin et al 303/21 P 3,583,7736/1971 Steinbrenner et al. 303/21 EB FOREIGN PATENTS OR APPLICATIONS1,953,253 6/1970 H Germany 303 2117 Primary ExaminerMilton BuchlerAssistant ExaminerStephen G. Kunin Attorney-Yount, Tarolli, Weinshenker& Cooper [57] ABSTRACT A compound anti-skid logic system in which aplurality of different logic circuits are operative in parallel toprocess input signals from a common source in accordance with thedifferent logic of each circuit with the different circuitscomplementing each other so that at least one logic circuit detects awheel lock or an impending wheel lock under widely variable roadconditions and brake application pressures. The compound system isincorporated in a master module with additional slave modules used withthe master module to control axles in addition to those controlled bythe master module.

7 Claims, 9 Drawing Figures Patented Oct. 23, 1973 3,767,270

7 Sheets-Sheet 'l POM/E //VVEN7'0A JOHN A. UREA/V 5y WWW FIG! PatentedOct. 23, 1973 '7 Sheets-Sheet 3 Palenled Oct. 23, 1973 7 Sheets-Sheet 4GMT Patented Oct. 23, 19.73

"/ Shoots-Sheet i,

Although the invention will be described with particualr reference to anair brake system for truck tractors and trailers, it will be appreciatedthat the principles of the invention have broader application and may beused with other types of vehicles and in brake systems other thanpneumatically actuated brake systems.

BACKGROUND OF THE INVENTION It has long been recognized that vehiclewheel lockup during braking produces several undesirable results amongwhich are the increased stopping distance required to halt the vehicle,increased tire wear and loss of operator control over the direction oftravel of the vehicle. To prevent the occurrence of results such asthese, a wide variety of anti-skid systems have been developed fordetecting wheel lock-up or impending wheel lock-up. Most, if not all, ofthese systems contemplate normal operator control of brake applicationuntil a locked or impending locked wheel is detected'by the system and,thereafter, the system automatically operates to control braking of thevehicle until the condition which triggered the system is removed.However, as a practical matter, the prior art systems have proven lessthan satisfactory due to, among other reasons, the numerous variablefactors which must be taken into consideration for any one system toperform satisfactorily under all possible conditions. Among thesefactors are the construction of the brake system, the inertia of thewheel and drive train assembly, the variable road conditionsencountered, the frictional aspects of the tires, the static brakeloading and the effect of weight transfers during braking.

In addition, it is highly desirable that the skid control system besufficiently sensitive to detect impending wheel locks so that actuallock-up of the wheel can be avoided. On the other hand, nuisanceactuation of the system due to normal braking and turning of thevehicleand different rolling radii of the tires must be avoided.

Another important consideration is that the integrity of the outputsignal from such systems should be maintained irrespective of themagnitude of the excess brake pressure applied to the brakes during askid condition. In other words, a skid signal should be generated by thesystem under conditions which may range from a very small excess brakeapplication to a very large excess brake application such as, forexample, where there is a panic stop on icy roads. It is to beunderstood that excess brake pressure is always present when a skidcondition occurs and it is the magnitude of the brake pressure in excessof the ideal brake pressure to which reference is made. The magnitude ofthis excess brake application is very small, a slow-lock may occur inwhich a wheel slowly decelerates to a lock-up condition without a skidcontrol signal being produced. Some skid control systems may allow thevehicle wheels to prematurely step down to zero speed; this step-lock,as it is called, may be occasioned in some systems during small excessbrake application pressure and in others during large excess brakeapplication pressures. in other systems, a heavy excess brakeapplication will result in a fast-lock of the wheels before the brakepressure can be relieved resulting in loss of the skid control signal.

SUMMARY OF THE INVENTION It is a primary object of this invention toprovide an improved anti-skid control system which is operative todetect a wheel lock or. an impending wheel lock and generate a skidsignal under virtually all operating conditions.

It is a more specific object of the invention to provide a compoundanti-skid logic system in which a plurality of different logic circuitsare operative in parallel to process input signals from a common sourcein accordance with the different logic of each circuit with thedifferent circuits complementing each other so that at least one logiccircuit detects a wheel lock or an impending wheel lock under widelyvariable road conditions and brake application pressure levels.

A further object of the invention is to provide a compound anti-skidlogic system which includes wheel and axle speed logic, wheel and axledeceleration logic and computed wheel and axle speed logic and which areall operative to produce a skid control signal for automatically varyingthe brake application pressure.

Still another object of the invention is to provide a skid controlsystem which readily lends itself to a modular system of packaging andin which a master module together with add-on slave modules may beutilized to provide skid control for'multiple'axle vehicles such astractor-trailer units or the like.

In accordance with the preferred form of the invention, the skid controlsystem comprises a master logic module which controls the brakeapplication pressure supplied to the two wheels on an axle with themodule including first logic circuit means for producing a skid signalwhen the two wheel speeds differ by a predetermined amount, second logiccircuit means for producing a skid signal when the deceleration ofeither wheel exceeds a predetermined threshold value and third logiccircuit means for producing a skid signal when the actual velocity ofeither wheel is less by a predeterminedamount than the computed velocityof the wheel under predetermined assumed conditions. The input signalsto each of the logic circuit means comprises first and second speedsignals representative, respectively, of the two wheels on the axle. ifa skid condition is detected by any one or more of the logic circuitmeans, a skid signal is generated to actuate a skid control valve whichrelieves the brake pressure and controls the subsequent application offluid pressure to the brakes. The valve comprises a compensating relayvalve which, upon actuation, rapidly vents the brake pressure and, uponrelief of the skid condition, reapplie's brake pressure first at a rapidrate up to a level close to but below the ideal brake pressure andthereafter at a slower rate until either the vehicle is stopped or askid condition is again detected and the valve is recycled.

In accordance with a further aspect of the invention it is contemplatedthat the skid control system may be used in a two axle vehicle with themaster module being associated with, for example, the steer axle of thevehicle and slave modules for controlling the brakes associated with thewheels on the other axle. The slave module receives two speed signalsrepresentative of the speeds of the two wheels with which that module isassociated. These two speed signals are applied to a first logic circuitmeans for generating a skid signal when the two wheel speeds differ by apredetermined amount. The slave module further includes second logiccircuit means for comparing the speed of the associated axle with theother axle of the vehicle and generates a skid control signal when thosetwo speeds differ by a predetermined amount. A third logic circuit meansis included in the slave module and operates to generate a skid controlsignal when the axle speed decreases at a rate in excess of apredetermined rate. The only input signal supplied to the first andthird logic circuit means in the slave module are the wheel speedsignals from the wheels of the axle associated with that module. Thesecond logic circuit means receives axle speed signals from the masterand slave modules.

This basic skid control system utilizing a master and a slave module maybe expanded by additional slave modules for each axle of the vehiclewith each additional slave module receiving an axle speed signal fromthe preceding slave module whereby a vehicle having almost anycombination of axles can be controlled.

Other objects, features and advantages of the invention will be moreapparent from the following description which, together with theattached drawings, discloses a preferred form of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings whereinlike reference numerals indicate like parts in the various views:

FIG. 1 is a schematic illustration ofa skid control system constructedin accordance with this invention incorporated in a two axle vehicle;

FIG. 2 is a chart illustrating the various combinations oftractor-trailer vehicles and the application of the skid control systemto those vehicles; 7

FIG. 3 illustrates in block diagram form one embodiment of the skidcontrol system having a master module and a slave module;

FIG. 4 is a schematic amplifier circuit for the master module of FIG. 3;

FIG. 4a is a schematic fixed bleed circuit;

FIG. 5 is a schematic amplifier circuit for the slave module of FIG. 3;

FIG. 6 is a schematic illustration, in block diagram form, of a modifiedslave module;

FIG. 7 is a cross-sectional view of a compensating relay valve which maybe incorporated in the skid control system of FIG. 1;

FIG. 8 is a graphic illustration of the pressure curves for each of thechambers in the valve of FIG. 7 during operation of the skid controlsystem.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more in detail tothe drawings, there is schematically illustrated in FIG. 1 a trucktractor having front steer wheels 10,12 carried on a front axis or axle,and rear drive wheels 14,16 carried on a rear axis or axle. The wheelsI0, 12, 14 and 16 are equipped with air brakes (not shown) which may beof conventional construction. Associated with the wheels are generatorsG1, G2, G3 and G4 of known construction which are driven in anyconventional manner in response to rotation of their respective wheelsto produce electrical signals proportional to the respective rotationalwheel speed. The speed signals produced by the generators G1 and G2 areapplied to a compound logic system 19 via conductors 20, 21 and 22,while the speed signals of generators of G3 and G4 are fed to thecompound logic system 19 via conductors 24, 25 and 26.

The air brake system associated with the vehicle includes a highpressure air reservoir 30 connected by a conduit 34 with a conventionaltreadle valve 32 which is operable by an operator controlled treadle 38to sup ply air pressure to a control conduit 36. The conduit 36 is influid communication with a pair of compensating relay valves 40, 42, thedetails of which will be described in greater detail hereinafter. Thevalves 40,42 function as relay valves during normal brake operation and,when a skid condition is encountered, as compensating skid controlvalves which complement the skid control provided by the logic system19. In the relay mode of operation, the valves 40 and 42 regulate themagnitude of air pressure supplied to the brakes in pro portion to themagnitude of deflection of treadle 38. The brake pressure for the frontwheels 10,12 is supplied from the reservoir 30 via conduits 44, 45,valve 40 and conduits 46,47. The brake pressure applied to the brakes ofwheels 14 and I6 is supplied via conduits 44, 48, valve 42, and conduits49 and 50.

If during braking a wheel-lock condition is detected, a skid signal isgenerated and applied to the appropriate valve 40 or 42, which willthereafter automatically control the applied brake pressure. Forexample, if the logic system 19 detects an incipient wheel-lockcondition related to left front wheel 12 and/or to right front wheel 14,a skid signal is generated which is applied via conductors.52,54 toenergize a solenoid 56 associated with valve 40. When thus actuated,valve 40 functions to block the flow of air to the brakes of wheels 12and I4 and vents the air in the system to atmosphere. When the skidcondition has been relieved, the solenoid 56 is de-energized and brakepressure is reapplied in a manner described in more detail hereinafter.

THE COMPENSATING VALVES The construction of the valves 40 and 42, whichare identical, is illustrated in FIG. 7. The valve 40 includes a valvehousing 60, a control diaphragm 62, a compensating diaphragm 64, anorifice and check valve assembly 66, and the solenoid valve 56. Thevalve 40 has five main chambers, namely, a control pressure chamber 68,a brake pressure chamber 69 separated from chamber 68 by the controldiaphragm 62, a high pressure chamber 70 communicating with chamber 69via a variable passage 71, a compensating chamber 72, and a biasingchamber 73 separated from the chamber 72 by the compensating diaphragm64. The control pressure chamber 68 is in communication with the controlpressure in conduit 36 via passages 74,75 and the normally open solenoidvalve 56. The solenoid valve 56 includes a coil 76, an exhaust passage77, a sliding plunger 78, and a spring 79 which normally biases theplunger 78 into a position blocking the exhaust passage 77.

The biasing chamber 73 communicates with the control pressure in thepassage 75 via a passage 30 and a bias orifice 82.

The compensating chamber 72 is in communication with brake pressurechamber 69 via passage 84 and 86. The orifice check valve assembly 66comprises a fill orifice in the form of a washer 87 and an exhaustorifice or washer 88. The opening in the exhaust orifice 88 issubstantially larger than the orifice opening in the fill orifice 87whereby fluid is admitted to chamber 72 at a rate controlled by therelatively small opening of the fill orifice 87. A spring 89 normallybiases the fill orifice 87 to the position shown in FIG. 7.

The valve 40 further includes a primary piston member 90 and a secondarypiston member 91. The primary piston member 90 has a disc portion 92disposed 'in the chamber 69 and engageable with the diaphragm 62. Theprimary piston 90 further includes a valve seal 93 engageable with avalve seat 94 to control the flow of fluid pressure between chambers 69and 70. g

The secondary piston member 91 includes a disc portion 95disposed inchamber 72 with the disc portion 95 being in engagement with thecompensating diaphragm 64.

The two piston members 90 and 91 are supported coaxially with a lightcoil spring 96 interposed between the adjacent ends of the two pistonsto bias the primary piston member upward and the secondary piston 91downward. The effective area of the diaphragm 64 which acts against thesecondary piston 91 is smaller than the effective area of the diaphragm62 which acts against the primary piston 90.

THE COMPOUND LOGIC SYSTEM Referring now to FIG. 3, one embodiment of acompound logic system 19 for a two-axle vehicle is shown in blockdiagram form. in this embodiment, the compound logic system 19 comprisesa master logic module 100 associated primarily with the vehicle fromsteer wheels 10,12 and a slave logic module 200 associated primarilywith the rear drive wheels 14,16. The master logic module 100 comprisesa pair of frequency-tovolt'age converters 102,104, a wheel speed logiccircuit 106 for comparing the speed of the front wheels 10,12, a wheeldeceleration logic circuit 108 for determining the deceleration of frontwheels 10,12 and comparing the deceleration with a reference, a summingamplifier 110, a computed speed logic circuit 112 for comparinginstantaneous axle speed with a computed axle speed based on an assumedpredetermined rate of deceleration of the axle, and a valve driver 114.When a skid signal is generated by any one of the logic circuits, thesignal is impressed on the valve driver 114 whichenergizes solenoid 56via wires 52,54.

The slave logic module 200 comprises a pair of frequency-to-voltageconverters 202,204, a wheel speed logic circuit 206 (identical to logiccircuit 106), a summing amplifier 208, an axle speed logic circuit 210for comparing the speed of the front steer axle and the rear drive axle,an axle deceleration logic circuit 212 for the rear drive axle, and avalve driver 214 for energizing the solenoid of valve 42.

While the compound logic system 19 has been illustrated as controlling avehicle having two axles, this same system may be adapted to controlvehicles having any number of axles simply by adding one slave logicmodule for each additional axle. FIG. 2 is a chart illustrating varioustypical types of vehicles and the makeup of the compound logic systemwhich would be applicable for each type of vehicle. it is to beappreciated that the master logic module and the slave modules used ineach of the combinations shown in FIG. 2 would be of the sameconstruction as described in detail hereinafter.

Considering now in more detail the master logic module and particularlythe wheel speed logic circuit 106, the operation of that logic circuitis predicated on the probability that the wheels on a common axis willnot decelerate at the same rate, due principally to differences in wheelload, brake effectiveness, and road-to-tire coefficients of friction.The wheel speed logic improves lateral vehicle stability during brakingby maintaining all wheels on a common axis within a predeterminedrolling speed range. The wheel speed logic circuit includes comparators116,118, both of which receive signals from the converters 102,104. Thecomparators 116 and 11'8function to compare the signals from theconverters 102 and 104,and depending on the difference in wheel speeds,a skid control Signal will be generated by one of the comparators andapplied to the output line 120.

The wheel deceleration logic 108 functions to detect excessive wheeldecelerations, which, if not corrected, would lead to a wheel lock-up.This logic has the ability to provide an early indication of animpending wheel lock. This circuit comprises two differentiators 122,124and two comparators 126,128. The differentiator 122 receives a signalfrom the converter 104 while the differentiator 124 receives its signalfrom the converter 102. The two differentiators differentiate theirrespective input speed signals with respect to time to produce a signalwhich is representative of the deceleration of the associated wheel. Thedeceleration signals generated are compared in comparators 126,128 witha reference signal. If either comparison indicates a skid condition, askid signal is generated and applied to the output line 120. I

The computed speed logic circuit is predicated on the fact that animpending locked wheel is indicated when the actual wheel velocity isless than the computed wheel velocity, assuming a deceleration potentialof the vehicle on a high coefficient surface. The circuit 1 12 comprisesa fixed bleed circuit 130 and a comparator 132. The summing amplifierreceives the wheel speed signals from converters 102,104 and combinesthese signals to produce an output signal which is applied both to thefixed bleed circuit and the comparator 132. The fixed bleed circuitstores the input signal and produces an output signal proportional tothe input signal as long as the input signal is increasing, constant, ordecreasing at a rate not greater than a predetermined rate. However, ifthe input signal decreases at a rate' greater than the predeterminedrate, the output of the fixed bleed circuit will decrease at a ratewhich maintains the output signal at a value which is a percentage ofwhat the average axle speed signal should be based on a predeterminedassumed deceleration. This output from the fixed bleed circuit 130 isapplied to the comparator 132 which compares it with the signal from thesumming amplifier 110 and, depending on the comparison, a skid signalwill be generated and applied to the output line 120.

P16. 4 illustrates an amplifier circuit which may be used for the mastermodule 100. The DC signals generated by the generators 61,62 are fed tothe converters 102,104. The output of the left signal converter 102 isfed to an inverter 134 while the output from the right converter 104 isfed to an inverter 1136 which change the signal polarity from negativeto positive. From the inverters the signals are fed to the wheel speedlogic circuit 106 which is embodied in the circuit of FIG. 4.

Conductors 138 and 139 apply the signals to a low gain differenceamplifier 140. The difference amplifier performs a subtracting functionto produce either a positive or negative output which is applied tocomparators 142,144. To avoid nuisance actuation of the system resultingfrom differences in wheel speeds which are not a result of a skidcondition, such as may result from normal braking or turning of thevehicle, a threshold value in the form of a reference voltage VC isapplied to each of the comparators 142,144. The reference voltages VCrepresent a threshold value which must be exceeded before eithercomparator 142 or 144 will producean output signal. Thus, if the outputof the difference amplifier 140 is negative, and is made ofa magnitudewhich exceeds the reference voltage VC, a positive voltage will beproduced by the comparator amplifier 142 which will pass through adiode146 to the output line 120. At the same time, the negative voltageapplied to the comparator 144 will result in a negative voltage outputfrom the comparator which is blocked by the diode 148. Conversely, ifthe output of the difference amplifier 140 is positive and in excess ofthe reference voltage VC the output of the amplifier 144 will bepositive and transmitted to output line 120 while that of the amplifier142 will be negative.

The threshold value represented by the reference voltages VC is anallowed difference between the compared speeds, which difference whenexceeded is deemed to justify generation of a skid signal. Thisthreshold value preferably varies with changes in vehicle speed so thatsuitable means may be employed to vary the magnitude of the referencevoltage depending on the vehicle speed. A

The wheel deceleration logic circuit 108 is embodied in the amplifiercircuit of FIG. 4 and includes low speed cut-out devices 150,152 whichrender the wheel deceleration function inoperative if the input signalsare below a certain predetermined level, such as 5 mph or less. Signalsrepresentative of speeds greater than 5 mph pass through the cut-outs150,152 to a pair of differentiators 154,156 which differentiate theinput speed signals with respect to time to produce a negativedeceleration signal with the signal from the amplifier 154 beingindicative of the deceleration of the left wheel and the signal producedby the amplifier 156 being indicative of the deceleration of the rightwheel 12. The signals from the differentiators 154,156 are applied tocomparator amplifiers 158,160 respectively. As with the comparatoramplifiers 142,144, a reference voltage VR is applied to the amplifiers158,160 with the applied reference voltage representing a thresholdvalue which must be exceeded at any given vehicle speed before a skidsignal will be generated. This threshold value is normally selected tobe greater than the vehicle deceleration obtainable on a highcoefficient surface. The deceleration signal fed to the comparatoramplifier 158 results in an output voltage which is proportional to thedifference of the input voltages. If the comparator voltage is negative,it is blocked by the diode 162 but, if positive, the voltage passesthrough the diode to the output line 120. The comparator amplifier 160operates in precisely the same manner as amplifier 158.

A threshold value of l g represented by the reference voltage VR hasbeen found to provide adequate warning of incipient wheel lockconditions and adequate time to initiate corrective action before anactual wheel lock occurs. However, the threshold value may be greater orless than 1 g depending on the type of brake system and vehicle use.Moreover, the threshold value varies with respect to the speed of thevehicle so that suitable means should be employed to vary the referencevoltage VR as a function of vehicle speed.

Referring now to the computed speed logic circuit 112 which is embodiedin the circuit of FIG. 4, the output signals from the converters 102,104are directed to a summing junction 166 and amplifier 168 where theoutputs are summed and fed through an active low pass filter 170 to thefixed bleed circuit 130 and to the negative terminal of comparator 172.The comparator 172 is similar to each of the comparators 142, 144, 168and 160. A suitable fixed bleed circuit 130 is shown in FIG. 4a. Asshown, the speed signal from filter 170 is fed into the base of anemitter follower 180. Capacitor 182 is charged through R1, diode 184 andR2 to the emitter voltage of 180. At constant speed or duringacceleration, capacitor 182 is prevented from discharging since diode186 is reverse biased. Upon wheel deceleration, diode 186 is forwardbiased, diodes 184 and 187 are reverse biased and the capacitordischarges through a constant current source 188. The rate of dischargeis controlled by R3 and is chosen to correspond to a predetermineddeceleration. The voltage at the junction of diodes 186 and 187decreases almost linearly, because the current drawn by resistor R4 isnegligibly small in comparison with the constant current in R3. Currentin R4 is therefore an approximate measure of voltage on capacitor 182,and serves to sense the voltage on the capacitor.

In the absence of the input signal decreasing at a rate greater than apredetermined rate, the fixed bleed circuit will produce an outputsignal which is a predetermined percentage of the input signal. As aspecific example, the fixed bleed circuit may be designed to produce anoutput signal which is percent of its input signal so long as the inputsignal is increasing, constant or decreasing at a rate not greater thana rate corresponding to a 0.9 g deceleration (0.9 g being the potentiallimit for a heavy duty truck). However, if the input signal decreases ata rate greater than a 0.9 g deceleration the circuit will maintain theoutput signal at a value representative of 80 percent of what theaverage axle speed signal should be based on a 0.9 g deceleration. Thus,during modes of operation when the average axle speed signal isincreasing, constant or decreasing at a rate representative of less thana 0.0 g deceleration, the input signal to the positive terminal of theamplifier 172 will be less than the input signal to the negativeterminal of the amplifier and a negative output voltage will be producedwhich will be blocked by the diode 174. However, for an average axlespeed deceleration in excess of 0.9 g there will occur, if thedeceleration time span is sufficiently long, a point where the outputsignal of the fixed bleed circuit will equal or begin to exceed theaverage axle speed signal. At this point in time, the average axle speedis 20 percent below the speed it should be, based on a 0.9 gdeceleration rate. As long as the signal at the positive terminal ofamplifier 172 is equal to or greater than the signal at the negativeterminal, the amplifier 172 will produce a positive voltage that istransferred to the output line 120.

it should be noted that each of the described skid control logiccircuits is ineffective under certain braking conditions to produce askid signal. However, it has been discovered that under virtually allbraking conditions at least one of the logic circuits will be operativeto produce a skid signal of the required integrity. Thus, byincorporating the individual logic circuits into a compound system withthe individual circuits operating independently and in parallel, skidcontrol is provided for the vehicle irrespective of the conditionsencountered.

The following chart summarizes in tabular form the quality of the skidcontrol signal generated by each of 10 the individual logic circuitsunder various conditions of excess brake application. The portionsenclosed in a solid box indicate an unacceptable signal, the portionenclosed in a dashed box represents an acceptable signal and the unboxedportions represent a desirable, l5

high quality signal.

1% iiJs'kid condition occurs, 'ifiepi'ssiire int he brake pressurechamber 69 will build up to the level of the pressure in the controlchamber 68 and act against the disc portion 92 of the primary piston 90to counterbalance the pressure forces exerted by the control diaphragmthrerby enabling the spring 96 to return the primary piston to a closedposition blocking off further flow of air to the brake pressure chamber.Upon release omefireadlefthe strains'asntrsraaaatrts will be vented backthrough the treadle valve 32 in a conventional manner thereby creating apressure imbalance across the control diaphragm 62. The higher pressurein the brake pressure chamber 69 will, by openings 98, act against theunderside of the diaphragm oz to deflect it upward to the dotted lineposition shown in FIG. 7 and thereby move the diaphragm away from anannular LO GTC INIE GRITY Excess lnnko application lmcclumtion logicSpeed logic Computed speed logic Very small A wheel can slowlydccelcrate to lock- Good signal No signal unless high rate of decelera-I up without producing a signal. tion takes place.

Small Early indication of impending lock-up Late indication of impendinglock-up? No signal unless high rate of deceleragives smooth stopwithoutlock-up. H leads to momentary look. i tion takes place. With lowvehicle deceleration, wheel references to high vehicle decelerationresulting in step-lock.

Medium Loss of signal during wheel roll-up Brakes are not reapplieduntil wheel Brakes are not reapplied until wheel causes prematurereapplication rehas rolled up to optimum speed. has rolled up tocomputed speed sulting in step-lock. Maintains signal even duringmowhich is less than optimum, resultmentary lock-up. ing in step-lock.

Heavy Exces br ke application locks wheels The excess application locksthe In the event of no other signal, th before the excess air can beexwheels before the excess air can be computed vehicle speed logic willhausted from the chambers. When exhausted. The speed reference willrelease the 'brakes at least two the wheels lock, deceleration is zerobe to locking wheels causing step cycles and in so doing, one or moreand the signal disappears. lock or all wheels may reach zero of theother logics will become efiecsimultaneously resulting in loss of tiveand the system will regain true signal. vehicle reference.

Unloaded full treadle on Excess brake application locks wheels Theexcess application locks the ln the event of no other signal, the

before the excess air can be exwheels before the excess air can becomputed vehicle speed logic Will hausted from the chambers. Whenexhausted. All wheels will reach release the brakes at least two i thewheels lock, deceleration is zero zero simultaneously resulting in loss1 cycles and in so doing, one or more i and the signal disappears. ofsignal. i of the other lcgics will become efiec- 1 i tive and the systemmay regain I Thus, the compound logic system l9 provides a skid controlsystem that (l) detects impending wheel lock early in the brake cycle(2) maintains a skid signal as long as the wheel speeds (or the axlespeeds) differ by an amount in excess of a predetermined thresholdvalue, and (3) maintains a skid signal long enough to insure brakerelease in the event that all wheels lock before the brake pressure hasexhausted sufficiently to permit wheel roll-up.

OPERATION The compound logic system 19 cooperates with the valve 40during a typical skid control cycle in the following manner. When thebrakes are initially applied by the operator depressing treadle 38,control air pressure rapidly rises in to act on the control diaphragm 62and the primary piston 90 to move the piston 90 downwardly, unseatingthe valve communicating chambers 69 and 70. The high pressure air fromthe reservoir 30 present in the high pressure chamber 70 is communicatedto the brakepressure chamber 69 and applied to actuate the brakes of thewheels 10 and 12. Assuming lffhowever, a skid condition is encount eredas, for

example, where the ideal brake pressure for the road conditions is 25psi and the control pressure is established by the operator at 60 psi,one or both of the wheels l0, 12 will begin to decelerate to a lockedcondition. Upon this occurrence, one or more of the logic circuits willdetect the skid or incipient skid and produce an output signal on theoutput line K20 which in turn energizes the solenoid 56 to pull theplunger 78 down against the bias of the spring 79 thereby venting thecontrol chamber 69 to atmosphere through the exhaust port 77. As thecontrol chamber pressure vents,

' a force difference is created across the diaphragm c2 which moves thediaphragm upwardly and allows thepressure in the brake pressure chamber69 to vent to atmposhere through the exhaust passage 99. When the brakepassage has been reduced sufficiently to allow the wheel which generatedthe skid signal to roll up to vehicle speed, the skid signal disappearsand solenoid 56 is de-ener'gized whereby control pressure is againadmitted to control chamber 68.

At this point in the braking cycle, the compensating skid aspects ofvalve 40 come into play. More particularly, from the time the vehicleoperator first initiated braking, pressure had been flowing to biaschamber 73 through the bias orifice 82 and passage 80. At the same time,brake pressure had been flowing to the compensating chamber 72 throughthe compensating fill orifice 87 and passages 84,86. The bias orifice 82and the compensating fill orifice 87 are so sized that the pressure inthe chambers 73,72 rises at substantially the same rate so that the netforce on the secondary piston 91 is substantially zero during initialbrake application. However, when the brake pressure chamber 69 is ventedto atmosphere, a pressure differential is created across the valveassembly 66 with the higher pressure in the compensating chamber 72acting against the fill orifice 87 to move it upward against the bias ofthe spring 89 thereby allowing the discharge from the compensatingchamber 72 to be governed by the relatively large opening in thecompensator exhaust orifice 88. At the same time, control pressure,which has been vented from the control chamber 68, continues to flow tothe bias chamber 72 from passage 75 thereby creating a pressuredifference across the bias diaphragm 64, moving the secondary piston 91upward against the bias of the spring 96 into intimate contact with theprimary piston 90.

When the brakes are reapplied, upon termination of the skid signal, theupward force on the secondary piston results in a brake pressure risecurve having a bend or knee as shown in the graph of FIG. 8. As shown inthat graph, the brake pressure rise curve upon reapplication has a firststage rapid rise rate to a level below the ideal brake pressure levelfollowed by a substantially slower rate of pressure rise. Thischaracteristic knee effect obtainable with the valve 40 increases theeffectiveness of the braking system by (1) allowing initialreapplication 1 pressure to rise rapidly and unre stricted so thateffective braking force is rapidly reestablished and (2) decreasingbrake pressure overshoot which decreases the number of skid cycles perunit of time. Moreover, the capability of decreasing brake pressureovershoot eliminates the necessity for the compound logic system tooperate, in subsequent cycles, in an excessively large brake applicationmode. Any subsequent skid conditions which occur as the brake pressurerise curve passes beyond the ideal brake pressure curve generates only asmall excess brake application pressure and assures generation of a skidcon trol signal of good quality.

The compound logic system cooperates with the valve 40 to continuecycling of the valve 40 in the manner illustrated in FIG. 8 until a skidcondition is nolonger detected and the vehicle is brought to a smoothstop.

SLAVE MODULE Turning now to the slave logic module 200 of FIG. 3, thatmodule cooperates with the valve 42 to control the wheels 14,16 insubstantially the same manner as'the master module 100 cooperates withvalve 40. The wheel speed logic circuit 206 includes comparators 216,218which function in precisely the same manner as the comparators 116,118to compare the wheel speed signals from wheels 14 and 16 and impress askid signal on valve driver 214 when the signals differ by an amount inexcess of the threshold value.

The axle speed logic circuit 210 is predicated on the probability thatall axles of a vehicle will not decelerate at the same rate, dueprincipally to differences in axle load, brake effectiveness androad-to-tire coefficients of friction. This logic circuit includescomparators 220 and 222 with the comparator 220 receiving the summedsignal from the summing amplifier 208 and, by

conductor 224, a signal from the summing amplifier 110 of the mastermodule. It will be appreciated that the summed signals from amplifiers110 and 208 are indicative, respectively, of the average axle speed ofthe front and rear axles. The same two signals are also applied to thecomparator 222. The comparator 222 compares the rear axle signal to thefront axle signal and if the rear axle signal exceeds the front axlesignal, produces an output skid control signal that is transmitted viaconductor 226 to the output line 120 of the master module where it isimpressed on the valve driver 1 14. Similarly, comparator 220 produces askid control signal when the front axle speed exceeds the rear axlespeed with the skid control signal generated by the comparator 220 beingtransmitted via the output 219 to the valve driver 214.

It is to be noted that the axle speed logic circuit 210 is the onlylogic circuit which requires a speed signal from any other module. If,in addition to the slave module 20, a second module is used to control athird axle,

the second slave module would receive its axle speed signal from theslave module 200.

The axle deceleration logic circuit 212 is basically the same as thewheel deceleration logic circuit 108 of the master module differing onlyin that the sum of the 5 wheel speeds are used rather than individualwheel -sentative of the deceleration of-the axle. This decelerationsignal is then compared in a comparator 232 to a reference signal. Ifthe comparison indicates a skid condition, a skid signal is generated onthe output line 219.

FIG. 5 is a schematic amplifier diagram similar to the diagram of FIG. 4and which may be used for the slave module 200. Several aspects of theslave amplifier circuit are similar both in function and construction tothe master module amplifier circuit so that only that portion of thecircuit which materially differs will be discussed.

The wheel speed logic circuit 206, as embodied in FIG. 5, comprises aninverter 233, a summing junction 234, a difference amplifier 236,comparator amplifiers 238,240 and a pair of blocking diodes 242,244. Thesignals applied to the summing junction 234 are opposite in sign so thatthe output will be the difference of the two signals and will have thesign of the larger. If the signals are the same magnitude, there is nooutput and consequently there is no output produced by the differenceamplifier 236. The comparators are connected to a reference voltage VCwhich represents a threshold value as discussed in connection with themaster module. When a negative signal is applied to the differenceamplifier 236, a positive signal is produced that causes comparator 240to produce a positive voltage which is supplied to the output line 219.When the output of the difference amplifier 236. is negative, comparatoramplifier 238 produces a positive voltage which is supplied to theoutput line 219.

The axle speed logic circuit 210, as embodied in FIG. 5, comprises adifference amplifier 246, two comparator amplifiers 248,250 and twoblocking diodes 252,254. The operation of this portion of the amplifiercircuit is, in all respects, analogous to operation of the wheel speedlogic circuit portion of the master module amplifier circuit.

The axle deceleration logic circuit 212, as embodied in FIG. 5, receivessignals which have been summed and amplified by a summing amplifier 256.The summed signals are directed to a low speed cut-out 258 and, ifgreater than the set value of the cut-out, enter a differentiater 260which differentiates the speed signals with respect to time to producean axle deceleration signal. This deceleration signal is applied to thecomparator amplifier 262 for comparison with a reference signal VC. Ifthe output of the comparator amplifier 262 is a negative voltage, it isblocked by the blocking diode 264 but if positive, a skid control signalis applied to the output line 219.

MODIFIED SLAVE MODULE FIG. 6 illustrates a modified slave logic modulein which a computed speed change logic circuit 270 has been added to thewheel speed logic circuit 206, axle speed logic circuit 210 and axledeceleration logic circuit 212. Logic circuit 270 includes a fixed bleedcircuit 272 and a comparator amplifier 274. This logic circuit isstructurally and functionally the same as logic lar reference tospecific embodiment, neither the illustrated embodiments nor theterminology employed in describing them is intended to be limiting;rather, it is intended to be limited only by the scope of the appendedclaims.

Having thus described the invention, what is claimed is:

1. An anti-skid control system for a vehicle having first and secondbrake equipped independently rotatable wheels on a common axis, saidsystem comprising:

valve means for applying said releasing fluid forces to the brakes,

first means operative during application of said brakes to generate askid signal when the difference between the rotational speeds of saidwheels exceeds a predetermined amount,

second means operative during application of said brakes to generate askid signal when the deceleration of either of said wheels exceeds apredetermined deceleration,

means responsive to a skid signal from either of said first and secondmeans for actuating said valve means to relieve the brakes associatedwith both of said wheels,

third means operative during application of said brakes to generate askid signal for actuating said valve means to release the brakesassociated with both of said wheels when the average rotational speed ofsaid wheels differs from a reference speed,

said vehicle including third and fourth independently rotatable brakeequipped wheels mounted in spaced relation on a second common axisspaced from the first axis,

valve means operative to apply and release fluid forces to the brakes ofsaid third and fourth wheels,

four means for producing a skid signal for actuating said valve meansassociated with the wheels of one of said axes when the average wheelspeeds on said one of said axes is less than the average wheel speeds onthe other of said axes by a predetermined amount, and

fifth logic means for producing a skid signal when the averagedeceleration of said wheels on said second axis exceeds a predetermineddeceleration value.

2. The skid control system of claim 1 and further in cluding means forproducing from the wheel speeds of the wheels on said second axis acomputed reference speed signal representative of wheel speed during apredetermined deceleration, and

sixth logic means for producing a skid signal when the average wheelspeed of said wheels on said second axis differs from said computedreference speed.

3. The skid control system of claim 1 and further including meansoperative to produce a skid signal when the difference between therotational speeds of said third and fourth wheels exceeds apredetermined amount.

4. A brake control system for a vehicle having a plurality of brakeequipped wheels mounted in spaced relation for rotation about a commonaxis, said system comprising,

valve means for applying fluid forces to actuate said brakes under thecontrol of an operator and for relieving said brake forces in responseto a skid signal,

signal generating means associated with each of said wheels to generatesignals representative of the speed of each wheel,

first logic means operative to generate a skid signal when said twowheel speed signals differ by a predetermined amount,

second logic means operative to receive said speed signals and togenerate a skid signal when the deceleration of either wheel exceeds apredetermined deceleration,

third logic means operative to receive said wheel speed signals and togenerate a skid signal when the wheel speed signal decreases below acomputed reference speed, and

circuit means interconnecting said logic means with said valve meanswhereby a skid signal generated by any of said logic means is operativeto actuate said valve means to relieve the brakes to all of said 5. Thebrake control system of claim 4 wherein said valve means includes meansfor restricing the reapplication of fluid forces upon removal of a skidsignal to an initial rapid rise in fluid forces followed by a subsequentslower rise in fluid forces.

6. An anti-skid control system for a vehicle having first and secondbrake equipped independently rotatable wheels on a common axis, saidsystem comprising:

valve means for applying and releasing fluid forces to the brakes,

first means operative during application of said brakes to generate askid signal when the difference between the rotatational speeds of saidwheels exceeds a predetermined amount,

- second means operative during application of said ond wheels forproducing first and second signal,

representative respectively of said first and second wheel speeds,

means for providing from said first and second speed signals an averagewheel speed signal having a magnitude representative of the averagespeed of said first and second wheels,

reference signal means for producing a computed reference speed signaland which comprises means for receiving said average wheelspeedsignal-and producing a reference signal which decreases in magnitude ata predetermined decay rate from an initial value representative of wheelspeed, and means for comparing said average wheel speed signal with saidcomputed reference speed signal and providing a skid signal whenever themagnitude of the average signal is less than that of said referencesignal. 7. An anti-skid control system for use with a vehicle having atleast first and second spaced apart, independently rotatable wheelswhich rotate on a common axis with each of said wheels beingequippedwith fluid operated brakes and a braking system for applyingfluid forces to each of said brakes, said braking system in-. cludingvehicle operator control means for controlling the application of fluidforces to the brakes and relay valve means actuated by said operatorcontrol means for applying or releasing fluid forces simultaneously toeach of the'brakes, said skid control system comprising:

generating means associated with said first and second wheels forproducing first and second signal representative respectively of saidfirst and second wheel speeds, means for providing from said first andsecond speed signals an average wheel speed signal having a magnituderepresentative of the average speed of said first and second wheels,reference signal means for producing a computed reference speed signaland which comprises means for receiving a speed signal from the wheelson said axis and producing a reference signal which decreases inmagnitude at a predetermined decay rate from an initial valuerepresentative of wheel speed, first means for comparing said averagewheel speed signal with said computed reference speed signal andproviding a skid signal whenever the magnitude of the average signal isless than that of said referencesignal, 7 I second means for providing askid signal when the deceleration of at least one of said wheels differsfrom a reference value, and means responsive to a skid signal fromeither of said first and second means for actuating said relay valve torelieve the fluid pressure applied to the brakes at both of said wheels.

1. An anti-skid control system for a vehicle having first and secondbrake equipped independently rotatable wheels on a common axis, saidsystem comprising: valve means for applying said releasing fluid forcesto the brakes, first means operative during application of said brakesto generate a skid signal when the difference between the rotationalspeeds of said wheels exceeds a predetermined amount, second meansoperative during application of said brakes to generate a skid signalwhen the deceleration of either of said wheels exceeds a predetermineddeceleration, means responsive to a skid signal from either of saidfirst and second means for actuating said valve means to relieve thebrakes associated with both of said wheels, third means operative duringapplication of said brakes to generate a skid signal for actuating saidvalve means to release the brakes associated with both of said wheelswhen the average rotational speed of said wheels differs from areference speed, said vehicle including third and fourth independentlyrotatable brake equipped wheels mounted in spaced relation on a secondcommon axis spaced from the first axis, valve means operative to applyand release fluid forces to the brakes of said third and fourth wheels,four means for producing a skid signal for actuating said valve meansassociated with the wheels of one of said axes when the average wheelspeeds on said one of said axes is less than the average wheel speeds onthe other of said axes by a predetermined amount, and fifth logic meansfor producing a skid signal when the average deceleration of said wheelson said second axis exceeds a predetermined deceleration value.
 2. Theskid control system of claim 1 and further including means for producingfrom the wheel speeds of the wheels on said second axis a computedreference speed signal representative of wheel speed during apredetermined deceleration, and sixth logic means for producing a skidsignal when the average wheel speed of said wheels on said second axisdiffers from said computed reference speed.
 3. The skid control systemof claim 1 and further including means operative to produce a skidsignal when the difference between the rotational speeds of said thirdand fourth wheels exceeds a predetermined amount.
 4. A brake controlsystem for a vehicle having a plurality of brake equipped wheels mountedin spaced relation for rotation about a common axis, said systemcomprising, valve means for applying fluid forces to actuate said brakesunder the control of an operator and for relieving said brake forces inresponse to a skid signal, signal generating means associated with eachof said wheels to generate signals representative of the speed of eachwheel, first logic means operative to generate a skid signal when saidtwo wheel speed signals differ by a predetermined amount, second logicmeans operative to receive said speed signals and to generate a skidsignal when the deceleration of either wheel exceeds a predetermineddeceleration, third logic means operative to receive said wheel speedsignals and to generate a skid signal when the wheel speed signaldecreases below a computed reference speed, and circuit meansinterconnecting said logic means with said valve means whereby a skidsignal generated by any of said logic means is operative to actuate saidvalve means to relieve the brakes to all of said wheels.
 5. The brakecontrol system of claim 4 wherein said valve means includes means forrestricing the reapplication of fluid forces upon removal of a skidsignal to an initial rapid rise in fluid forces followed by a subsequentslower rise in fluid forces.
 6. An anti-skid control system for avehicle having first and second brake equipped independently rotatablewheels on a common axis, said system comprising: valve means forapplying and releasing fluid forces to the brakes, first means operativeduring application of said brakes to generate a skid signal when thedifference between the rotatational speeds of said wheels exceeds apredetermined amount, second means operative during application of saidbrakes to generate a skid signal when the deceleration of either of saidwheels exceeds a predetermined decleration, and means responsive to askid signal from either of said first and second means for actuatingsaid valve means to relieve the brakes associated with both of saidwheels, generating means associated with said first and second wheelsfor producing first and second signal representative respectively ofsaid first and second wheel speeds, means for providing from said firstand second speed signals an average wheel speed signal having amagnitude representative of the average speed of said first and secondwheels, reference signal means for producing a computed reference speedsignal and which comprises means for receiving said average wheel speedsignal and producing a reference signal which decreases in magnitude ata predetermined decay rate from an initial value representative of wheelspeed, and means for comparing said average wheel speed signal with saidcomputed reference speed signal and providing a skid signal whenever themagnitude of the average signal is less than that of said referencesignal.
 7. An anti-skid control system for use with a vehicle having atleast first and second spaced apart, independently rotatable wheelswhich rotate on a common axis with each of said wheels being equippedwith fluid operated brakes and a braking system for applying fluidforces to each of said brakes, said braking system including vehicleoperator control means for controlling the application of fluid forcesto the brakes and relay valve means actuated by said operator controlmeans for applying or releasing fluid forces simultaneously to each ofthe brakes, said skid control system comprising: generating meansassociated with said first and second wheels for producing first andsecond signAl representative respectively of said first and second wheelspeeds, means for providing from said first and second speed signals anaverage wheel speed signal having a magnitude representative of theaverage speed of said first and second wheels, reference signal meansfor producing a computed reference speed signal and which comprisesmeans for receiving a speed signal from the wheels on said axis andproducing a reference signal which decreases in magnitude at apredetermined decay rate from an initial value representative of wheelspeed, first means for comparing said average wheel speed signal withsaid computed reference speed signal and providing a skid signalwhenever the magnitude of the average signal is less than that of saidreference signal, second means for providing a skid signal when thedeceleration of at least one of said wheels differs from a referencevalue, and means responsive to a skid signal from either of said firstand second means for actuating said relay valve to relieve the fluidpressure applied to the brakes at both of said wheels.